The invention relates to a tone receiver comprising a zero crossing detector which includes an amplifier circuit that has an inverting input and a non-inverting input. Such receivers are known, for example, from the article entitled "A monolithic dual tone multifrequency receiver" in IEEE Journal of Solid State Circuits, Vol. SC-14, No. 6, December 1979. These tone receivers may be used in telephone sets for detecting tone signals customary in telephony, such as DTMF signals (Dual Tone Multi Frequency).
Amplifier circuits having an inverting and a non-inverting input have the property of being capable of presenting an offset voltage i.e. it is possible that a voltage unequal to zero occurs at the output if there is a zero voltage difference between the two inputs. This offset voltage has a detrimental effect on the operation of the zero crossing detector, because the zero level of the amplifier/comparator input (that is, the input voltage which produces a zero voltage on the output) now no longer has a symmetrical position between the positive and negative input threshold voltages to be set. This may have the effect that the amplifier circuit acting as a comparator is energized by small interference signals which are smaller than the set threshold value.